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mrbluze writes "The standard kilogram weights used by countries around the world for calibration have variably increased in mass by tens of micrograms. This poses a threat to the precision and comparability of measurements in science, engineering and trade. The problem is due to surface contamination, but a safe method of cleaning the weights has only recently been devised by the use of ozone and ultraviolet light (abstract). 'The ultraviolet light-ozone treatment removes hydrocarbon contamination that has built up on the metal surface, gunk that comes from the emissions of an industrial society. Cumpson suspects that because the kilos living in national labs have been retrieved and handled more frequently than the international kilo, more carbon-containing contaminants have built up on them over time. Incubating the kilograms with a set amount of ozone and ultraviolet light "gently breaks up the carbonaceous contamination at the surface."'"